Since the advent of the assembly line, conveyors and methods of conveying a workpiece through a series of stations have been instrumental in manufacturing processes. Numerous classes of conveyors exist, including overhead conveyors, such as trolley conveyors and power-and-free conveyors, flight conveyors, screw conveyors and the like. Most include one or more continuouslydriven chain and carrier attachments defining a drive course and a return course. The carrier attachments or tracks which guide the wheels or a carrier such as a wheeled pallet may be an open beam, semi-enclosed channel or a combination of these structures. Vertical track bends typcially link the drive and return courses through which the carrier may be propelled by rotating paddles.
One specific type of conveyor, known in the art as an accumulating pallet chain-dog conveyor, includes a power-driven endless chain to which longitudinally spaced pusher dogs or other devices are attached. The dogs are adapted for operative pushing engagement with an assembly which is attached to the underside of a pallet for advancing the pallet along the drive and return courses. In conveyors of this type, a single endless chain, centrally disposed with respect to the dog-engaging pallet assembly, continuously moves through the drive course and the return course. The dog-engaging assembly on the underside of the pallet engages one of the dogs and is thereby carried through the conveyor course. A pallet stop is provided which, when activated, engages the pallet, halting its movement through the drive course. The dog-engaging assembly on the underside of the pallet release the dog in response to the pallet's engagement with the pallet stop. Additional pallets then accumulate behind the first or primary stopped pallet in the same fashion until the pallet stop is retracted. Each dog-engaging pallet assembly in turn then reengages a passing dog whereby the flow of the pallets through the drive course is reestablished.
The chain-dogs, or "dogs", which are attached to the endless chain are longitudinally spaced with a plurality of chain-spacers or "spacers" being attached to the endless chain between the dogs. Thus, in operation at a given point in the conveyor course a dog passes followed by a plurality of spacers with another dog following the spacers in a continuous manner. Each dog has a front or leading portion which is expanded laterally such that the dog resembles a wedge. In accumulating pallet chain-dog conveyors, the dog-enegaging assembly on the underside of each pallet includes a pair of opposed, spring-biased circular rollers which, when a dog passes therebetween, engages the leading, expanded portion of the dog such that the dog carries the pallet through the course. When the forward movement of the pallet is stopped by a pallet stop, the continued forward movement of the dog causes the rollers to release the dog, whereby the spacers then move freely through the space between the opposed rollers. In these conventional conveyor systems, as the dogs move out of engagement with the opposed rollers, a space exists between the opposed rollers and the spacers such that there is no engagement therebetween. That is, the spacers are free to move between the rollers without providing any contact whatsoever between the rollers and the spacers. As the next dog approaches the stationary pallet, the leading edge of the dog engages the opposed rollers which then move apart, allowing the dog to pass therethrough, and once again the spacers which follow the dog pass freely between the opposed rollers.
This conventional arrangement has several disadvantages. Most notably, when the pallet first engages the pallet stop and releases the dog, the pallet is freely positioned on the track. There is therefore a corresponding rearward movement of the pallet in response to its impact with the pallet stop and the release of the dog. In the past, in order to prevent this unwanted rearward movement of the pallet, it was necessary to provide a second pallet stop to engage the rearward end of the pallet. Hence, as the pallet engaged the forward pallet stop it started to "bounce back" on the track whereupon it then engaged the rearward pallet stop so that the rearward movement of the pallet on the track did not continue. However, this rearward pallet stop still allowed some rearward movement of the pallet, making it difficult to precisely position the pallet and thus the workpiece which it carries. It is known that precise positioning of the workpiece is necessary in many automated manufacturing processes. For example, where the operation on the workpiece is performed by robots, precise positioning of the workpiece by the pallet is generally required. Therefore, this pallet movement between the pallet stops or "chatter" is highly undesirable.
It is also known that in conventional accumulating pallet chaindog conveyors, the pallets are propelled through the turn course by a rotating paddle which engages the bottom of the pallet at the ends of the drive course and return course. Often, a pallet which reached the end of the drive course remained there a period of time until the paddles rotated into position for engagement with the pallet. In these devices, the paddles typically comprised two radial spokes. Thus, in those instances where a pallet had to wait for the next spoke, valuable processing time was lost.
Therefore, it would be desirable to provide an accumulating pallet chain-dog conveyor which does not need a rearward pallet stop and which eliminates pallet chatter. It would also be desirable to provide such an accumulating pallet chain-dog conveyor in which the pallets are immediately propelled through the turn course without unduly awaiting the next spoke.
I have discovered such an accumulating pallet chain-dog conveyor and a method of conveyance which achieves these goals.